28. Why doesn't a long lasting
vaccine against common cold
exist yet?

Gap-fill exercise

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Viruses that present a mutation
rate like the that causes the
common escape easily from the
of vaccines against them. a
primary immune response ( or
artificially induced) against virus in
the next of infection new mutant
strains appear and the
obtained with the immune
of the last season lost. (One
could say the high mutation rate
a form of “immunization” by
these viruses.)